the log actually ends here (drm_ioctl does not return)
at the same time (until it freezes) Xorg echoes tons of this error message:
mieqEnequeue: out-of-order valuator event; dropping
tossed event which came in late
then the xserver lies mtsleeping():
629 Giulio 39 0 1900K 12M drmlk2 0:11 24.22% 19.58% Xorg

No idea about the error reason, neither if the interrupted ioctl locks
("returning EINTR")